1. Field of the Invention
The present invention relates to a technique for inspecting pattern on an object.
2. Description of the Background Art
In the field of inspection of pattern formed on a semiconductor substrate, a color filter, a shadow mask, a printed circuit board or the like, conventionally, a comparison check method has been mainly performed with gray-scale images. For example, a differential image which indicates absolute values of the difference between an inspection image (an image to be inspected) and a reference image is obtained and a region in the differential image which has pixel values larger than a predetermined threshold value is detected as a defect. In a case of inspection of pattern having periodicity, a plurality of inspection images are sequentially acquired and a comparison check is performed by using an inspection image other than the image under inspection as a reference image.
Such a comparison check has a problem that a threshold value to be determined is changed by variation in graininess of an image due to variation in sharpness. FIGS. 1A and 1B are graphs each showing a histogram of an absolute value of difference (hereinafter, referred to as “differential absolute value”) between pixels of the inspection image and corresponding pixels of the reference image (in other words, a histogram of differential image). FIG. 1A shows a histogram 91a in a case where the graininess of image is large and FIG. 1B shows a histogram 91b in a case where the graininess of image is small. FIGS. 1A and 1B show the histograms of the differential images which are obtained on the same pattern, but the distributions of differential absolute values in the histograms are different due to the difference in graininess of pickup images.
Therefore, for example, when a threshold value T1b is determined in accordance with the histogram 91b of FIG. 1B, if the graininess of the inspection image temporarily becomes large and the distribution of the differential absolute values comes into a state of FIG. 1A, a normal pixel (a pixel corresponding to a normal region) whose differential absolute value is larger than the threshold value T1b and smaller than the threshold value T1a is detected as a false defect. On the other hand, when the threshold value T1a is determined in accordance with the histogram 91a of FIG. 1A, if the graininess of the inspection image temporarily becomes small and the distribution of the differential absolute values comes into a state of FIG. 1B, a defective pixel (a pixel corresponding to a defective region) whose differential absolute value is larger than the threshold value T1b and smaller than the threshold value T1a is not detected.
Then, Japanese Patent Application Laid-Open Gazette No. 2002-22421 proposes a method for removing an effect of variation in sharpness of image (in other words, variation in graininess) by calculating a standard deviation of pixel values of the differential image and normalizing the histogram of the differential image on the basis of the standard deviation.
In a case of inspection of pattern in which an aluminum wiring whose surface has coarse grain is formed in a relatively flat region (hereinafter, referred to as “background region”) on a semiconductor substrate, if a histogram 92 of a differential image on a differential absolute value as shown in FIG. 2A is obtained, an appropriate defect detection can not always be performed even if a threshold value T2a is determined on the basis of the histogram 92.
The reason is that the histogram 92 of FIG. 2A is obtained by synthesizing a histogram 921 of a differential image on an interconnection region and a histogram 922 of a differential image on a background region as shown in FIG. 2B and the differential absolute value of a defective portion 922a in the histogram 922 on the background region becomes smaller than the threshold value T2a due to an effect of the histogram 921 on the interconnection region. As a result, even if the method disclosed in the Japanese Patent Application Laid-Open Gazette No. 2002-22421 is used, it is impossible to determine an appropriate threshold value in a case of the histogram 92 of FIG. 2A.